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Post-Boot Delays in SSH Access Due to Small Entropy Pool

posted by Fnord666 on Friday December 21 2018, @06:19PM   Printer-friendly
from the chaos-monkey dept.

canopic jug writes:

Recent upgrades that depend on the new Linux getrandom() syscall can cause OpenSSH to delay starting for tens of minutes while waiting for enough bytes of randomness. There are currently not any feasible work-arounds.

Systemd makes this behaviour worse, see issue #4271, #4513 and #10621.
Basically as of now the entropy file saved as /var/lib/systemd/random-seed will not - drumroll - add entropy to the random pool when played back during boot. Actually it will. It will just not be accounted for. So Linux doesn't know. And continues blocking getrandom(). This is obviously different from SysVinit times when /var/lib/urandom/random-seed (that you still have laying around on updated systems) made sure the system carried enough entropy over reboot to continue working right after enough of the system was booted.

#4167 is a re-opened discussion about systemd eating randomness early at boot (hashmaps in PID 0...). Some Debian folks participate in the recent discussion and it is worth reading if you want to learn about the mess that booting a Linux system has become.

While we're talking systemd ... #10676 also means systems will use RDRAND in the future despite Ted Ts'o's warning on RDRAND [Archive.org mirror and mirrored locally as 130905_Ted_Tso_on_RDRAND.pdf, 205kB as Google+ will be discontinued in April 2019].

Related post: OneRNG: a Fully-Open Entropy Generator (2014)

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  • (Score: 0) by Anonymous Coward on Friday December 21 2018, @08:46PM

    by Anonymous Coward on Friday December 21 2018, @08:46PM (#777313)

    And for virtual machines, a virtual hardware RNG, much like you get virtualised other hardware like NICs, TPM etc. VMs using this virtual RNG device can then use the hypervisor's pool as an additional entropy source (or whatever the hypervisor chooses to feed to the VM through this interface). Of course there'd need to be safeguards in the hypervisor to ensure there isn't a denial-of-service on the host if one VM suddenly asks for a huge amount of entropy in a short period of time, or a large amount of requests from lots of machines.